Ink jet printing is a standard and preferred method for printing onto a substrate, wherein a stream of ink droplets are directed from a printing device to a surface of the substrate. The direction of the stream is controlled electronically in causing the droplets to print the desired image or information on the substrate surface without requiring contact between the printing device and the surface to which the ink is applied. Ink jet printing may be applied to a variety of substrates including, but not limited to, metals, glass, synthetic resins, plastics, rubber, paper, and the like. Objects, comprising substrates to which ink jet printing is well suited, include, but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, bank cards (debit, credit, and the like), identification strips (e.g., comprising barcodes), and the like.
Fluorescent inks have been developed for printing a “security mark” on articles such that the mark is invisible to the unaided eye, but that can be detected as fluorescence upon excitation with an activating light of a suitable excitation wavelength spectrum. More particularly, security marks are applied to articles in efforts to prevent forgery, theft, and fraud; wherein such articles are known to include, but are not limited to, identification cards, passports, currency, checks, securities, and other types of commercial paper. The security mark may be in the form of a barcode which encodes information, or may comprise a recognizable pattern for identification and verification purposes. Prior art fluorescent inks are known in the art to include: a fluorescent colorant, a solvent, and a binder resin; an organic laser dye that is excited by a wavelength spectrum in the near infrared range and fluoresces in an infrared range; a phenoxazine derivative dye fluorescing in the near infrared range; a rare earth metal, an organic ink carrier, and may further comprise a chelating agent; and a near infrared fluorophore and a water-dissipatable polyester. Generally, such fluorescent inks comprising fluorescent dyes or pigments, present with several disadvantages. For example, there is a relatively narrow margin between the amount of a fluorescent dye which is sufficient to give good fluorescent color intensity, and an amount wherein the dye molecules begin to aggregate and thus reduce the amount of fluorescence by quenching. A limitation in intensity can also limit the density of information encoded on a security mark (such as a barcode) by a fluorescent ink composition.
Thus, there exists a need for fluorescent ink compositions suitable for printing on substrates, wherein (a) the fluorescent component of the fluorescent ink composition is water-soluble; (b) the fluorescent component is excited by a wavelength spectrum comprising UV light, and preferably in a spectral range of from about 300 nanometers (nm) to about 400 nm, and emits an narrow emission peak in a wavelength spectrum primarily in the visible range, and preferably in a spectral range of from about 410 nm to about 750 nm; (c) a plurality of fluorescent ink compositions (each containing a fluorescent component that can be detectably distinguished (e.g., by fluorescent color and/or intensity) from that of other fluorescent ink compositions of the plurality of fluorescent ink compositions) may be utilized for multicolor fluorescence by excitation with a single wavelength spectrum of light resulting in simultaneous detection of fluorescence of high quantum yield and with discrete peak emission spectra; (d) a fluorescent component that resists photobleaching (and therefore can be used for signal integration); and (e) a fluorescent component that is not susceptible to quenching.